Firejail

Firejail is an easy to use SUID sandbox program that reduces the risk of security breaches by restricting the running environment of untrusted applications using Linux namespaces, seccomp-bpf and Linux capabilities.

Installation

Add following line to your system configuration to install Firejail globally

environment.systemPackages = with pkgs; [ firejail ];

You can also use the Firejail NixOS module for a persistent usage of specific applications which should always run in Firejail. The following example wraps the browser Librewolf and the messenger Signal in a Firejail environment. The usual program path to librewolf and signal-desktop will be overwritten by the Firejail-wrapper.

programs.firejail = {
  enable = true;
  wrappedBinaries = {
    librewolf = {
      executable = "${pkgs.librewolf}/bin/librewolf";
      profile = "${pkgs.firejail}/etc/firejail/librewolf.profile";
      extraArgs = [
        # Required for U2F USB stick
        "--ignore=private-dev"
        # Enforce dark mode
        "--env=GTK_THEME=Adwaita:dark"
      ];
    };
    signal-desktop = {
      executable = "${pkgs.signal-desktop}/bin/signal-desktop --enable-features=UseOzonePlatform --ozone-platform=wayland";
      profile = "${pkgs.firejail}/etc/firejail/signal-desktop.profile";
      extraArgs = [ "--env=LC_ALL=C" "--env=GTK_THEME=Adwaita:dark" ];
    };
  };
};

Usage

To start an application in a sandboxed enviroment use Firejail like this

firejail bash

For a graphical application like Firefox web browser, it is recommended to also use a profile

firejail --profile=$(nix --extra-experimental-features nix-command --extra-experimental-features flakes eval -f '<nixpkgs>' --raw 'firejail')/etc/firejail/firefox.profile firefox

Tips & tricks

Torify application traffic

The following example configuration creates a virtual network bridge which can be used in Firejail as an isolated network namespace. All traffic originating from this interface will be routed through a local Tor service which will therefore anonymize your internet traffic.

services.tor = {
  enable = true;
  openFirewall = true;
  settings = {
    TransPort = [ 9040 ];
    DNSPort = 5353;
    VirtualAddrNetworkIPv4 = "172.30.0.0/16";
  };
};

networking.bridges."tornet" = {
  interfaces = [];
};
networking.interfaces.tornet.ipv4.addresses = [{
  address = "10.100.100.1";
  prefixLength = 24;
}];

boot.kernel.sysctl = {
  "net.ipv4.conf.tornet.route_localnet" = 1;
};

networking.firewall.extraCommands = ''
  iptables -t nat -A PREROUTING -i tornet -p udp -m udp --dport 53 -j DNAT --to-destination 127.0.0.1:5353
  iptables -t nat -A PREROUTING -i tornet -p tcp -j DNAT --to-destination 127.0.0.1:9040
  iptables -A INPUT -i tornet -p tcp --dport 9040 -j ACCEPT
  iptables -A INPUT -i tornet -p udp --dport 5353 -j ACCEPT
'';

Run your preferred application inside the isolated Tor network

firejail --net=tornet --profile=$(nix --extra-experimental-features nix-command --extra-experimental-features flakes eval -f '<nixpkgs>' --raw 'firejail')/etc/firejail/firefox.profile firefox

Please note that this is a experimental setup which doesn't guarantee anonymity or security in any circumstances.